Method of removing deposited carbon from a thermal cracking apparatus



METHOD OF REMOVING DEPOSITED CARBON FROM A THERMAL CRACKING APPARATUS v Filed July L2. 1967 54 7 HIROHARU SHIMIZU ETAL 3,532,542

United States Patent or ice 3,532,542 METHOD OF REMOVING DEPOSITED CARBON FROM A THERMAL CRACKING APPARATUS Hiroharu Shimizu, Kudamatsu-shi, and Kinichi Omura,

Tokuyama-shi, Japan, assignors to Idemitsu Petrochemical Co., Ltd., Tokyo, Japan Filed July 12, 1967, Ser. No. 652,970 Claims priority, application Japan, July 25, 1966, 41/ 48,307 Int. Cl. B08b 7/00 US. Cl. 134-20 2 Claims ABSTRACT OF THE DISCLOSURE Carbonaceous deposits are removed from a thermal cracking apparatus comprising a pyrolysis furnace having an internal piping, a transfer line exchanger, and a pipe connecting the outlet of the furnace with said exchanger by passing serially therethrough a mixture of steam and air immediately after the supply of raw material hydrocarbon to the furnace has been discontinued and without cooling the furnace. The combustion products are removed from the apparatus only at a position downstream from the transfer line exchanger.

The invention relates to a piping system for and a method of removing deposited carbon from a thermal cracking apparatus.

It is generally known that in the manufacture of ethylene or other similar primary products where a tubular furnace is used for the pyrolysis of a raw material hydrocarbon such as naphtha, deposition of carbon, usually referred to as coking, takes place within the internal piping and outlet pipe of the furnace as well as in the transfer line exchanger connected thereto. The term transfer line exchanger is used in the art to mean a rapidly cooling heat exchanger which is supplied with pressurized cold water and which also serves to produce steam. The deposition severely affects the operation of the apparatus so that it must be interrupted periodically in order to remove deposited carbon therefrom.

The usual remedy for this problem has been to mount a branched pipe to the outlet of the furnace for exhausting carbon oxide as carbon deposited on the inner wall of the furnace piping is burned off by oxidation with air, and also mechanical means have been used to drive carbon deposits from the outlet pipe of the furnace and from the inside of the transfer line exchanger. To this end, the pyrolysis furnace is usually allowed to cool to the point at which it is accessible to personnel for attachment of a blind plate to the outlet pipe and for opening a valve in the branched pipe connected to the outlet pipe of the furnace. Then the furnace is heated again and air is supplied thereto for burning carbon, whereafter, the combustion products are exhausted through the branched pipe. This process requires a considerable time period, because the usual operating temperature of the furnace is such that the outlet pipe will be subject to high temperatures on the order of 600700 C. and the furnace must be naturally cooled to avoid any thermal stress. Moreover, the cooling and heating cycle can cause fatigue of the material used for the piping within the furnace. On the other hand, mechanical scavenging of the oulet pipe of the furnace and the transfer line exchanger requires time for dismounting related parts and also causes material abrasion.

Because of the high temperature existing at the outlet pipe of the furnace, the provision of a valve in the outlet pipe is not desirable in respect of fusion of the valve seat. Thus it is apparent that there has been a need in the art for 3,532,542 Patented Oct. 6, 1970 an improvement in the method of removal of deposited carbon from thermal cracking apparatus.

Therefore, it is an object of the invention to overcome said disadvantages by providing a novel method for removing deposited carbon from thermal cracking apparatus.

According to the invention, a branched pipe is connected to the outlet of the transfer line exchanger and is closed by a valve during normal operation of the thermal cracking apparatus. When decoking is required, the supply of raw material to the apparatus is replaced by a supply of steam and air, the proportion of these components being controlled so that only steam is supplied during the initial phase and subsequently the proportion of air is increased up to The valve in the outlet pipe of the transfer line exchanger is closed, and the valve which normally blocks the branched pipe is opened to exhaust the combustion products from every part of the apparatus.

The invention will now be described more particularly with reference to the drawing which shows a systematic diagram of a piping system for carrying out the method.

Referring to the drawing, a pyrolysis furnace contains an internal piping 2 which is connected with a supply pipe 3 and with an outlet pipe 4. In one example, raw material to be pyrolyzed, for example, naphtha is supplied through the pipe 3 at a rate of tons per day and the pyrolysis product from the furnace 1 is passed through a transfer line exchanger 5 for heat exchange, and is then fed to a main fractionator which is not shown, via a pipe 6 and a valve 7 which is normally kept open. A branched pipe 8 is connected to the pipe 6, but is usually closed by a valve 9.

When decoking is desired, the supply of raw material through the inlet pipe 3 is replaced by a mixture of steam and air. The amount chosen is 100 tons per day of steam at a gauge pressure of 14 kg./cm. and 4000 cubic meters per day of air. The inlet of the internal piping 2 will be at about 200 C., while the outlet pipe will be subject to a high temperature between 650 C. and 700 C., since the temperature of the atmosphere within the furnace is in the range of 1000 to 1500 C. The valve 7 is closed and the valve 9 is open. The heating of the furnace 1 is interrupted, but the heat exchanger 5 may be conveniently kept in operation to dissipate heat carried by flow of combustion products therethrough which are at elevated temperatures. The heat exchanger 5 also serves to produce steam from cooling water. It will be appreciated that according to the invention, the whole apparatus, namely, furnace 1, outlet pipe 4 and the exchanger 5 are simultaneously decoked, with the combustion products being exhausted through the common exhaust or branched pipe 8. Thus, the decoking operation according to the invention is performed without the necessity of allowing the furnace to cool, dismounting the pipe connections and attaching a blind plate to the outlet pipe of the furnace, or mechanically scavenging the exchanger and the outlet pipe, with the consequence that the rate of thermal cracking operation is considerably improved. To give an example, when a Selas type pyrolysis furnace used to produce ethylene from naphtha supplied at a rate of 130 tons per day and its auxiliary equipment were decoked in the prior art manner as set forth, it took about six days between interruption of operation until the apparatus was again ready for normal operation. However, according to the method of the invention, the inoperative time was reduced to only two days.

Upon completion of the removal of deposited carbon from the apparatus, the valve 9 is closed and the valve 7 is opened, and the apparatus is then once again ready for normal operation.

What is claimed is:

1. A method of removing carbonaceous deposits by oxidation from a thermal cracking apparatus of the type comprising a pyrolysis furnace having an internal piping, a transfer line exchanger, and a pipe connecting the outlet of the furnace with said exchanger, said outlet being at a temperature in excess of 600 during normal operation thereof, wherein the internal piping, the exchanger and the pipe all have carbonaceous deposits therein, said method comprising passing, immediately after the supply of raw material hydrocarbon to the furnace has been discontinued and without cooling the furnace, a mixture of steam and air through the furnace, the pipe and the exchanger in series to simultaneously combust said carbonaceous deposits formed in the internal piping of the furnace, the pipe and the exchanger, and removing the combustion products from the apparatus only at a position downstream from the transfer line exchanger.

2. A method as claimed in claim 1, in which the transfer line exchanger has an outlet means comprising a first valve controlled conduit for conducting pyrolysis products from the exchanger and a second valve controlled '4 branch conduit through which said combustion products are exhausted, the method further comprising closing the first conduit and opening the second conduit when exhausting the combustion products.

References Cited UNITED STATES PATENTS 1,470,359 10/1923 Greenstreet 134-22 1,919,672 7/1933 Thomas et al. 134-22 2,289,350 7/1942 Dixon et al 13420 2,289,351 7/1942 Dixon et a1 134-30 XR 2,537,079 1/1951 Nicolai et a1 13439 XR 2,577,254 12/1951 Lawson 134--22 XR 2,671,741 3/1954 Duvall 134-22 XR 3,054,700 9/1962 Martin 134-2 MORRIS O. WOLK, Primary Examiner J. T, ZATARGA, Assistant Examiner US. Cl. X.R. 134-2, 39 

